Direct Chip Attach (DCA) or flip-chip bonding is used in the semiconductor industry to connect a semiconductor die to a next level of interconnect wiring such as a ceramic chip carrier, or an organic printed circuit board. One method of DCA, known As Controlled Collapsed Chip Connection (C-4), involves depositing high lead content solder bumps on wettable bonding pads of a semiconductor die. These solder bumps are then soldered to traces or pads on the next level interconnect such as a PC board.
Before the solder bumps can be connected to the PC board, the PC board must be prepared by placing a low temperature solder, such as a eutectic tin-lead solder, on pads located on the PC board where physical and electrical connections are desired. Subsequently, the solder bumps of the device are aligned on the solder coated pads and heated such that the eutectic solder creates a connection between the device, C-4 bump, and PC board. Joining of the C-4 solder bumps directly to the PC board requires high temperatures, greater than 330.degree. C., and PC board materials able to withstand these temperatures, such PC boards are generally too expensive for most applications.
A problem with C-4 technology in DCA applications is the use of a second low temperature solder on the PC board. Placing the eutectic solder on a printed circuit board in order to accommodate the attachment of a C-4 die requires additional time and cost. It has been estimated that the additional cost of placing the eutectic solder on a printed circuit board can cost in the range of $0.50 to $1.00 per DCA chip. This additional cost is prohibitive for some applications. Attempts have been made to overcome the costs of applying eutectic to the circuit board by placing the eutectic on the C-4 die bumps. While this eliminates the cost of applying eutectic to the circuit board, it still requires the entire C-4 process to be completed before applying the eutectic, and is an additional step in the C-4 process. Yet another disadvantage associated with the use of the C-4 structures, is the cost of forming the high lead content material by evaporation techniques. Therefore, it has been illustrated over time that the use of C-4 bump structures tend to be costly in the manufacturing environment.
Yet another long standing problem with the use of C-4 bumps is device reliability over time, especially when joined to the next level of interconnect with high tin solders. It has long been observed, that under certain conditions the under bump metalization (UBM) is attacked causing reliability issues. In extreme cases, portions of the UBM entirely lift off of the die, and into the bump itself. The result is that the high lead bump is then in direct contact with the chrome layer, which does not provide for a good intermetalic interface.
Another type of DCA utilizes an Evaporated, Extended Eutectic process (E-3). An E-3 bump structure includes a thin tin layer, or cap, formed directly atop a substantially thicker lead layer. By using the tin cap, the bump structure, when heated, forms a eutectic liquidous layer by reacting with a small portion of lead that makes up the bulk of the evaporated bump. Use of an E-3 bump eliminates the need of preparing the PC board with eutectic solder. Furthermore, use of E-3 bumps eliminates the need to reflow the solder bump prior to attachment to the next level substrate.
While the use of E-3 bump structures has overcome some disadvantages of C-4 structures, the E-3 bumps are problematic in that they are relatively soft. E-3 bumps are relatively soft because of the thick lead layer. Lead is a highly ductile element. While the ductility of lead has certain advantages, there are other instances where highly ductility is undesirable. For example, highly ductile bumps are more susceptible to deformation by physical force, such as occurs during packaging and shipping of devices. Once damaged, subsequent processing can not be guaranteed. Therefore, once an E-3 bump structure is deformed, the device needs to be discarded.
A growing field of interest in the DCA industry is the use of eutectic bumps in order to overcome the problems of the prior art. However, the use of eutectic bumps has proven to be problematic as well. One problem associated with eutectic bumps on DCA devices has to do with limitations of the organic circuit boards to which the bumped die are attached. Generally, printed circuit boards, especially in low cost applications, have wide manufacturing margins for defining pad interconnect locations. As a result of these wide tolerances, substantial amounts of copper interconnect on a PC board may be exposed as a contact site. During the attachment of a eutectic bumped die structure to such a circuit board, the wetting between the solder, associated with the bumps of the DCA die, and the copper interconnect on a PC board are such that the resulting standoff height, which is the distance from the surface of the PC board to the surface of the device, is below a dimension from which currently available underfill processes can reliably be used.
One prior art technique used to overcome the problem of minimal standoff heights, is to form of copper standoffs for the die which limit the height to a specific distance. However, problems with the use of copper standoffs also exist. One problem with the use of copper standoffs is that large copper standoffs are capable of transferring stresses into active portions of the die which result in reliability failures. Conversely, small copper standoffs can be problematic in that they react with the tin resulting in a copper standoff becoming completely reacted and therefore resulting in a less reliable connection.
Therefore, it would be useful to identify a bump structure capable of use in DCA applications that overcomes problems of the prior art.